1. Field of the Invention
The present invention relates to a method for controlling read-time of a humidity sensor in a microwave oven, and more particularly to a method capable of controlling read-time of a humidity sensor in a microwave oven so as to avoid interference by other parts in the microwave oven, and thereby sensing an accurate humidity value in the microwave oven.
2. Description of the Prior Art
In general, automatic cooking methods used in the microwave oven are classified largely into a program type automatic cooking method and a sensor type automatic cooking method. According to the sensor type automatic cooking method, a humidity sensor and others in the microwave oven sense the humidity value of the moisture emitted from food put and heated in a heating chamber and other surrounding conditions such as temperature, and the cooking is controlled by automatic setting of the heating time according to the humidity sensing. Therefore, a great deal of research and development has been concentrated on the way to sense the humidity.
A conventional humidity sensing circuit used for a microwave oven as described above will be described hereinbelow with reference to the FIG. 1.
In FIG. 1, reference numerals 1, 2, 3, and 4 respectively designate a humidity sensing section for sensing humidity, an amplifying section for differentially amplifying the output of humidity sensing section 1, a microcomputer for outputting a control signal for controlling the humidity according to the amplified signal from amplifying section 2, and an equilibrium control section for controlling the equilibrium of humidity sensing section 1 according to the humidity control signal from microcomputer 3.
The humidity sensing circuit shown in FIG. 1 operates as follows.
At an initial stage of sensing the humidity, since an exact read-time for sensing humidity has not yet been set, microcomputer 3 sends an humidity control signal having a predetermined value through output terminals P0 through P4 and equilibrium control section 4 and thereby presets humidity sensing section 1.
Then, voltages of two nodes a and b of humidity sensing section 1 are inputted through non-inversion terminal and inversion terminal of an amplifier OPl of amplifying section 2 and then are differentially amplified. The amplified voltages are inputted through an humidity value input terminal A/D of microcomputer 3. In this case, the inputted voltages corresponding to the humidity value have analog forms, and are inputted into an analog/digital converter in microcomputer 3 and then converted to digital values.
However, such values as above are different from the practical humidity value. It is because, in a microwave oven, food is heated by a microwave generated by a magnetron installed in the microwave oven so that leakage of the microwave, which necessarily happens in the course of heating the food, has an effect on the humidity sensing performed by humidity sensing section 1.
Hereinafter, more detailed description about the above-mentioned leakage of the microwave will be given with reference to FIG. 2 for showing a general control circuit of a microwave oven in relation to the operation as described above.
As shown in FIG. 2, the control circuit of the microwave oven comprises a low voltage transformer 11 and a high voltage transformer 12 for elevating the voltage of the inputted electric power to a predetermined value, an amplifying section 13 connected to high voltage transformer 12 so as to amplify the elevated voltage, a magnetron for generating a microwave utilizing the voltage amplified in amplifying section 13 as a driving power, a power supply section 15 for supplying an electric power into a control circuit board using the voltage received from low voltage transformer 11, an interrupt signal generating section 16 for generating an interrupt signal according to the power supply from power supply section 15, an humidity sensing section 17 for sensing the humidity and an amplifying section 18 for amplifying the sensed humidity value, and a microcomputer 19 for generally controlling various parts in microwave oven according to signals received form interrupt signal generating section 16 and amplifying section 18.
When the control circuit of microwave oven is operated, an input power of 110/220 V and 60 Hz is firstly applied to high voltage transformer 12 according to the control by door switch and relay switches for driving the magnetron which are not shown. Then, a voltage elevated to about 2000 V is applied from the second windings of high voltage transformer 12 to amplifying section 18 having a high voltage condenser H.V.C. and a high voltage diode H.V.D. and then is doubled to about 4000 V. The doubled voltage is applied to magnetron 14 as a driving voltage so as to make magnetron 14 oscillate a microwave. At this time, since the electric current is interrupted during half-period die to the characteristic of the high voltage diode in amplifying section 18, an oscillation mode and a rest mode alternate corresponding to the frequency of the input power during the whole oscillation.
At the same time, the input power is supplied through low voltage transformer L.V.T. to power supply section 15 in the control circuit board, and power supply section 15 transforms the input power into a direct current power and then supplies the direct current power to microcomputer 19, humidity sensing section 17, and other load driving relays. The power supplied to microcomputer 19 passes through interrupt signal generating section 16 which applies an external interrupt signal as a pulse signal by a zero-crossing detection of frequency of an electric power of power supply section 15. Generally, interrupt signal generating section 16 is used in order for microcomputer 18 to determine whether the frequency of an electric power is a predetermined frequency such as 50 Hz or 60 Hz, or used for generating an interrupt signal for a specific abject such as time-count, in a conventional control circuit of a microwave oven.
As described above, in the conventional control circuit of the microwave oven, there is a possibility that the microwave oscillated in magnetron 14 leak and flow through wires and nodes into the circuit and then function as noise to the sensed humidity value.
To overcome the above described problem, various methods for minimizing the leakage of microwave have been proposed. An example of the methods is shown in FIG. 1 in which a plurality of noise-absorbing condenser C1 through C4 are connected to humidity sensing section 1 and amplifying section 2 so as to reduce the effect of the leakage of microwave.
However, just installation as such can not entirely remove the effect of the leakage of microwave. On the contrary, the voltage charged in the condensers can have bad effect on the sensed humidity value so as to cause the value more inaccurate. Further, the condensers necessarily invite increase in the number of parts of the circuit so as to make the circuit be more complicated.
Meanwhile, there is another method in which the humidity values are sensed several times and then a mean value of the sensed humidity values is adopted as a resultant sensed humidity value. However, neither this method can entirely remove bad effect by the leakage of microwave.